1. Field of the Invention
The present invention relates to a light-receipt system for use in optical digital communication having a light-receipt element, such as avalanche photo diode.
More particularly, it relates to a light-receipt system having a bias circuit, which can be controlled to have an optimum multiplication factor (M.sub.OPT), and which can realize to stably operate detecting the disconnection of a light input.
2. Related Art
According to that high speed and wide-band communication has been widely desired in recent year, optical digital communication has been used extensively. A general structure of a light-receipt system, that is used for the optical digital communication is as shown in the functional block diagram illustrated in FIG. 27.
It is general that an avalanche photo diode is used as a light-receipt element in the light-receipt system. In FIG. 27, reference numeral "10" is an avalanche photo diode (hereinafter, referred to as APD).
The bias current of the APD 10 is controlled by a bias control circuit 11. When a receipt light is inputted to the APD 10, the corresponding current flows as an electrical signal, and the current is converted to a voltage for leading to a preamplifier 12.
The electrical signal amplified in the preamplifier 12 is led to an identifying and reproducing and clock extracting circuit 14, via an equalizing amplifier 13. An identified and reproduced data DATA and an extracted clock CLK are outputted from the identifying and reproducing and clock extracting circuit 14.
On the other hand, an output of the equalizing amplifier 13 is also inputted to the peak detecting section 15, and the output of the equalizing amplifier 13 detected in the peak detecting section 15 is compared with a predetermined reference value in a comparator 16. In the case where the peak value of the output of the equalizing amplifier 13 is smaller than the predetermined reference value, the light input is judged as disconnected.
It is further desired in the light-receipt system, to have a wide dynamic range, especially, for the light input, and to operate detecting the disconnection of the light input, stably.
The former problem of the wide dynamic range for the light input is caused by saturation of the above-described preamplifier 12, deterioration of the APD operative range, and more particularly, the influence in the multiplication factor M at the minimum light input level and the maximum light input level.
Further, the latter problem that the light input disconnection can not be detected is caused because an amplifier having a high gain has to be mounted according to the demand for making the system compact. These problems will be further considered in detail as follows.
FIG. 28 is an example showing the circuit of trans-impedance type preamplifier employed as an preamplifier 12. In FIG. 28, a transistor TR1 and a transistor TR2 are connected in the form of cascade, a diode D1 and a resistor R2 are connected with an emitter circuit of the transistor TR2 in series, and an output voltage V.sub.OUT is outputted from the emitter of the transmitter TR2.
Further, a feed back resistor Rf is connected from the cathode of the diode D1 to the base side of the transistor TR1.
FIG. 29 is a characteristic chart of an input current versus an output voltage, indicating the relation between the input current I.sub.IN of the preamplifier 12, which is the output of the APD 10 and the output voltage V.sub.OUT of the preamplifier 12. Further, if the relation between the input current I.sub.IN and the output voltage V.sub.OUT will be considered referring to FIG. 28, it will be expressed as the following formulas. EQU V.sub.1 =V.sub.BE(TR1) ( 1) EQU V.sub.2 =V.sub.1 -I.sub.IN .times.R.sub.f ( 2) EQU V.sub.OUT =V.sub.2 +V.sub.D1 ( 3)
From these formulas (1) through (3), EQU V.sub.OUT =V.sub.1 -I.sub.IN .times.R.sub.f +V.sub.D1 ( 4)
Further, in the above-described formula, V.sub.ee =0 V to make the formula simple.
Hereupon, when the input level becomes larger according as I.sub.IN becomes larger, the current would not flow to the diode D1 and the resistor R2, so that the relation becomes V.sub.OUT =V.sub.D1.
Accordingly, the input current I.sub.IN(MAX) at the time the relation becomes V.sub.OUT =V.sub.D1 is called as input saturation current and expressed as the following formula. EQU I.sub.IN(MAX) =V.sub.1 /R.sub.f ( 5)
Further, the collector potential of the transistor TR1 is reversed to the base potential, and the transistor TR1 is saturated, because it becomes V.sub.2 =0 and the current source of the transistor TR2 and the diode D1 are lost, when the relation becomes I.sub.IN &gt;I.sub.IN(MAX).
Then, the characteristic of the input and the output becomes as shown in FIG. 29. A point P shows a saturation point in the diagram. When the transistor TR1 is saturated, it does not return to the feedback condition, until the charge accumulated in the parasitic capacity between the base and collector was fully discharged, even if the relation becomes I.sub.IN &lt;I.sub.IN(MAX).
Accordingly, the deterioration of the waveform response as the input and output waveform shown in FIG. 30 causes symbol error. That is, in FIG. 30, (1) shows a waveform of the input current I.sub.IN, and (2) and (3) are waveforms of the output voltage V.sub.OUT.
On the relation of the input and output between (1) and (2), the input current I.sub.IN (1) is small. The input light level is small according as the input current I.sub.IN (1) is small, so that the waveform of the output voltage V.sub.OUT (2) is not deteriorated, and thus the judging of the symbol can be performed correctly.
On the other hand, in the case of (3) in FIG. 30, it is a waveform of the output voltage V.sub.OUT in the case where the input light level is large and the input current I.sub.IN is close to the input saturation current I.sub.IN(MAX). In this case, the waveform is not returned to 0, and deteriorated by the saturation of the transistor TR1, until the charge accumulated in the parasitic capacity located between the base and collector was fully discharged, described above.
Accordingly, in this case, the symbol error in judging is caused, if it is identified at the time shown with the broken line in FIG. 30. Therefore, the saturation current I.sub.IN(MAX) is improved, when the V.sub.1 is made larger or the R.sub.f is made smaller, according to the formula (5). However, it is difficult to change the inside circuit, in the case where the circuit element (IC) commercially available in the market is used.
The above-described problem occurs in the case where the input current I.sub.IN of the preamplifier 12 is maximum, that is, the light dynamic range is at the maximum light-receipt level. On the other hand, the following problem occurs at the time the light dynamic range is at the minimum light-receipt level, as well.
FIGS. 31 and 32 are structural diagrams used as the prior art of the APD bias control circuit of the light-receipt system shown in FIG. 28. FIG. 31 is a diagram explaining a predetermined bias system which is one example of the conventional bias system.
In FIG. 31, reference numeral "10" is an avalanche photo diode (APD), and reference numeral "11" is a circuit for generating a predetermined bias voltage used as a bias control circuit. Further, reference numeral "12" is a preamplifier, of which output is led to an equalized amplifier 13 and the like connected backward, as shown in FIG. 28.
This is a basic bias circuit, however, the characteristic is deteriorated largely according to the variability of temperature and power, and the dispersion of avalanche photo diode, when the receipt-light power is minimum. And there is no control of gain of the APD, that is, a multiplication factor M, because it is a predetermined bias system, so that the light input dynamic range is narrow.
Thus, it is general to use the bias system having an AGC loop as shown in FIG. 32. FIG. 32 is a structural example of light-receipt system using the conventional bias system having the FULL-AGC loop, in contrast to a structural example of the general light-receipt system explained according to FIG. 27.
In FIG. 32, the light input signal is outputted after converting to the electrical signal by the APD 10. This output is led to the equalizing amplifier 13 via a preamplifier 12. And the waveform of the output is equalized in the equalizing amplifier 13 and led to the peak voltage detector 7.
The peak of the signal is detected in the peak voltage detector 7, and inputted to a bias circuit 11 constituted of a DC-DC convertor, via the amplifier 8. The size of bias is varied, corresponding to the peak detection signal to be inputted in the bias circuit 11. Hereby, the peak value of the output of the equalizing amplifier 13 is controlled so as to be constant.
Further, the output of the equalizing amplifier 13 is inputted to the identifying and reproducing circuit 14, and the data and clock are reproduced in this circuit 14. And it is also led to the timing extracting circuit 141, the timing signal is extracted in this circuit 141, and led to a light input disconnection detecting circuit.
The light input disconnection detecting circuit having a peak voltage detecting section 15 and a comparator 16 detects the state of the disconnection of the light input by detecting that the timing signal is disconnected for a given interval.
There is a problem on the above-described conventional bias circuit and system, as follows. Further, the problem will be explained referring to FIGS. 33 and 34. FIG. 33 is a diagram showing the relation between the conventional bias control system, that is, a multiplication factor control system, and the optimum multiplication factor.
In FIG. 33, the axis of abscissas shows a light input power, and the axis of ordinates shows the multiplication factor M. "P.sub.MIN " means a minimum light input power. Further, the characteristic of the optimum multiplication factor is shown with "M.sub.OPT ".
As is apparent from this diagram, the multiplication factor M is predetermined in the predetermined bias system shown in FIG. 31 (it is shown as the M-predetermined system in FIG. 33), and the multiplication factor M is always large for the characteristic of the optimum multiplication factor M.sub.OPT, so that the output is deteriorated by the saturation of the avalanche photo diode, at the time the light-receipt power is maximum.
On the other hand, the multiplication factor M becomes smaller as the light input power becomes larger, unlike the predetermined bias system, in the case of the bias system having FULL-AGC loop shown in FIG. 32. This is because the output amplitude of the equalizing amplifier 13 is controlled so as to be constant by an AGC feedback loop.
Hereupon, FIG. 34 showing the relation between the conventional APD output signal current and noise and the multiplication factor will be observed. In FIG. 34, the axis of abscissas shows the value of the multiplication factor M, and the axis of ordinates shows the APD output signal current i and the noise N.
The characteristic M.sub.OPT of the optimum multiplication factor is settled in the position where S/N ratio is the best, when the shot noise is minimum and the signal is maximum, as 2 shown in FIG. 34.
In the case of the bias system having the FULL-AGC loop, the multiplication factor M becomes smaller, corresponding to the increase of the light input, as the output signal current of the avalanche photo diode APD is controlled so as to be constant, even if the light input is increased as shown in 1 of FIG. 34 (FIG. 34 3).
Then, the shot noise becomes larger as shown in 4. However, the increase of the shot noise is smaller than that of the signal, and the input reduced noise becomes dominant. That is, in the case where the light input signal is made larger from the state of the optimum multiplication factor M.sub.OPT, the output current S of the avalanche photo diode APD and the noise N are constant, and the S/N ratio also becomes constant, so that error rate is predetermined (floor), without improving.
On the other hand, in the case of the fixed bias system, although both the output current S of the avalanche photo diode APD and the noise S become larger, according as the light input is made larger, the floor is not generated as the S/N is improved, because the noise N is increased with square root.
From the viewpoint of that, it is difficult to perform detection of the disconnection of the light input from the output current of the avalanche photo diode APD, in the case of the bias system having the FULL-AGC loop. That is, the bias current I.sub.APD of the avalanche photo diode is in the number .mu.A order, so that it is difficult to perform the detection of the light input disconnection by the variability, as the change of V.sub.APD bias is small, because the inclination of the characteristic of I.sub.APD -V.sub.APD is sharp.
Therefore, the light input disconnection detecting circuit for performing the detection of the disconnection of the timing signal is required, so that the circuit becomes complex.
Further, the response characteristic of the FULL-AGC loop is very slow and the circuit becomes complex, so that the problem upon which it is not easy to determine the time constant appears.
Accordingly, it is preferable to use the self-bias system. FIG. 35 shows a structure of the self-bias system.
As shown in FIG. 35, self-bias control resistors R1 and R2 are connected to the APD 10 in series, and the multiplication factor M is controlled by the self bias system. The relation between the light input power P.sub.IN in this case and the bias voltage V.sub.APD of the APD 10 in this case becomes as follows. EQU I.sub.APD =(e.multidot..lambda..multidot..eta.).div.(h.multidot.c).times.M.times.P.s ub.IN ( 6) EQU M=1/[1-(V.sub.APD /V.sub.B).sup.n ] (7) EQU V.sub.APD =V.sub.0 -(R1+R2).times.I.sub.APD ( 8)
Provided that I.sub.APD : an average current of the APD 10, e: electron charge, .lambda.: input wavelength, h: Plank's constant, c: speed of light, .eta.: quantum efficiency of the APD 10, M: multiplication factor of the APD 10, P.sub.IN : average light input power, V.sub.APD : bias voltage of the APD 10, V.sub.B : breakdown voltage of the APD 10, V.sub.0 : self-bias control voltage, R1 and R2: self-bias control resistors and n: index number of the APD multiplication factor determined by elements.
From the formula (8), the bias voltage V.sub.APD is reduced as shown in FIG. 36, according as the I.sub.APD at the maximum light-receipt level is increased. The frequency band is reduced as shown in FIG. 37, when this V.sub.APD is smaller than the band deteriorated voltage determined by elements, and becomes 10 MHz at the maximum light-receipt level.
Therefore, an error according to an intersymbol interference is generated in the input signal. As the resistance values of the resistors R1 and R2 are made smaller to keep the band of the APD, by making the value of the V.sub.APD larger, the multiplication factor M becomes larger according to the above-described formula (7), and the I.sub.APD is increased. Thus, this brings a problem that the light input level becomes lower to saturate and the preamplifier 12 connected backward to make the dynamic range of the preamplifier 12 narrower.
Hereupon, the above-described light input disconnection detecting circuit composed of the above-described peak detecting section 15 and a comparing amplifier 16 will be further considered. The peak detecting section 15 realizes to detect the disconnection of the light input by detecting the peak of the output of the equalizing amplifier 13.
In FIG. 38 for showing the output waveform characteristic of the equalizer with respect to the P.sub.IN, V.sub.P1 and V.sub.P2 have the input level of the preamplifier 12, that is, the size corresponding to the light-receipt level of the APD 10. V.sub.P1 shows the minimum light-receipt level (1), and V.sub.P2 shows the level at the time the light input is disconnected.
The difference between the input signals V.sub.P1 and V.sub.P2 is minute, so that the difference of the peak voltage enough to be able to detect the disconnection of the light input in the comparator 16 can not be obtained without making the gain of the amplifier connected backward or the equalizing amplifier 13 large enough.
On the other hand, the light-receipt system is required to make compact, so that it brings the following problems, in the case where the amplifier having high gain is mounted to solve the above-described problem.
At first, the oscillation according to the leak in the light-receipt system is generated. That is, it brings a problem that the disconnection of the light input can not be detected because of providing the impedance between the power source pattern (V.sub.CC, V.sub.ee) and the ground (the case ground) and generating the oscillation due to the leak between the input and output of the amplifier having a high gain.
At second, the oscillation is generated at the time the light-receipt system is mounted to a mother board. That is, it brings the problem that the capacitance according to the interval space between the bottom of the light-receipt system and the signal and power source pattern of the mother board near the system, and the inductance component of interface ground pins connecting the light-receipt system to the ground form a resonant circuit, and the amplifier having a high gain is oscillated with the resonant frequency of the resonant circuit.